A direct estimate of (REF ) from particle (sample) data is given by [1]}:
\(\gamma _k^2(f,g)&=\frac{1}{m^2}\sum _{i,j=1}^mk(X_i,X_j)-\frac{2}{mn}\sum _{i,j=1}^{m,n}k(X_i,Y_j) \\&+\frac{1}{n^2}\sum _{i,j=1}^nk(Y_i,Y_j), \) 
